The prevalence of opioid use disorder (OUD) during pregnancy has more than doubled in the past decade, as has maternal and neonatal morbidity and mortality related to OUD. Since the beginning of the opioid epidemic, maternal deaths attributable to drug overdose have increased by nearly 5-fold. In 2012, one infant, on average, was born every 30 minutes in the United States with neonatal abstinence syndrome (NAS), which has accounted for an estimated $1.5 billion in healthcare expenditures. In clinical trials, medication-assisted treatment (MAT) use during pregnancy has been associated with reductions in adverse maternal and neonatal outcomes. Despite this, many women with OUD do not receive treatment during pregnancy and the majority discontinue treatment in the postpartum period. Therefore, the goal of this R01 proposal is to understand how three different modifiable components of the delivery of OUD treatment services (i.e. availability, accessibility and structure) differentially impact maternal and child health outcomes. To accomplish this goal, we will utilize administrative healthcare data including claims, encounters, and pharmacy data for all Medicaid beneficiaries in Pennsylvania including >13,000 pregnant women with OUD and their infants, from 2008 to 2017. Data will be merged with birth certificate and early intervention data to determine if improvements in maternal OUD treatment engagement impact pediatric health outcomes. We will create patient-level measures of OUD treatment availability (geographic proximity to Medicaid OUD treatment providers), accessibility (duration of Medicaid enrollment) and treatment structure (guideline concordance). The primary research aims are to 1) assess the impact of OUD treatment availability during pregnancy (geographic proximity to Medicaid OUD treatment providers) on maternal MAT use and birth and neonatal health outcomes;? 2) assess the impact of OUD treatment accessibility (duration of maternal Medicaid enrollment) on MAT use during pregnancy and postpartum and pediatric health outcomes;? 3) determine if the relationships between MAT use and maternal and child health outcomes are mediated by OUD treatment structure (concordance with OUD treatment guidelines in pregnancy). Deeply affected by the opioid epidemic, Pennsylvania has the fourth-largest Medicaid program, rural and urban populations and demographic and socioeconomic profiles consistent with national averages which will provide highly generalizable findings. Aim 1 will quantify the minimum geographic proximity to Medicaid OUD treatment providers necessary to improve MAT use during pregnancy and subsequent birth outcomes. Aim 2 will determine the relevance of pre-pregnancy and postpartum Medicaid enrollment to first trimester MAT use and pediatric health outcomes for the first three years, which will inform Medicaid enrollment criteria. Aim 3 will quantify the extent to which recommended components of OUD treatment structure, beyond MAT use, are associated with maternal and child health outcomes. Together, our findings will provide high-quality evidence about how Medicaid policies may lessen the considerable burden of OUD.